Electrode Rolling Process and Key Control Factors

I. Working Process

Electrode rolling press machine is generally completed by a double-roll press, which consists of two cast steel compaction rolls, a motor, and a transmission shaft. When the press is not in use, a thin oil layer should be applied to prevent rust. Before use, the oil layer is wiped off with absolute ethanol, and the unwinding and rewinding mechanisms, automatic edge correction mechanism, rolls, and other components that may come into contact with the electrode and potentially contaminate it are cleaned. The coated electrode is fixed on the unwinding mechanism, passed through the gap between the two rolls, and connected to the rewinding system. After activating the rolling mode, the motor drives the upper and lower rolls to rotate simultaneously. The rewinding mechanism pulls the electrode to steadily pass through the rolling gap, and finally, the sheet is compressed to the required compaction density.

II. Purpose of Rolling

The ultimate goal of rolling is to obtain electrodes that meet design requirements. What does "meeting design requirements" mean? It refers to ensuring that relevant parameters of the electrode—such as areal density, compaction density, adhesiveness, particle integrity, and warpage—meet the design specifications of the cell.
Necessity of rolling: After coating and drying, the peel strength between the active material and the current collector foil is very low. Rolling is required to enhance the adhesive strength between the active material and the foil, preventing peeling during electrolyte immersion and battery use.

III. Problems During Rolling and Solutions

1. Electrode Fracture

During rolling, the electrode may suddenly break, which not only affects work efficiency but also causes troubles in subsequent processes such as cutting and winding.
Causes:
① If small particles or uneven textures remain on the electrode surface during coating, these particles will be pressed toward the foil by the rolls during rolling. Softer particles may be crushed into powder and fall off, while harder particles can puncture or even break the foil.
② If the areal density of the electrode surface is uneven during coating, over-rolling may occur in one area and under-rolling in another during rolling. Under the same tension control during sheet feeding, insufficient rolling can cause partial shedding of active materials or even foil breakage.
Solutions: Control the rewinding tension and prevent large particle impurities from falling onto the electrode surface to effectively reduce fractures.

2. Severe Electrode Warpage

After rolling, the electrode may exhibit significant concave or convex deformation, which is unfavorable for cutting and winding. A critical issue in the coating process is "thick edges," which is a major cause of warpage after rolling. As the edge thickness is several to tens of microns greater than the middle part, the thicker edge area bears higher rolling pressure, leading to inconsistent transverse compaction density and severe warpage. This also affects subsequent cutting processes.
Solutions: Controlling the coating quality of the electrode is key. Adjusting parameters such as slurry surface tension, pump pressure, feeding speed, and rolling pressure (within design limits) can effectively reduce warpage.

3. Uneven Transverse and Longitudinal Thickness

Transverse Thickness Variation: During rolling, the left and right thickness of the electrode may be inconsistent. If the pre-rolled sheet has uniform left-right thickness, the rolling pressure should be adjusted laterally to ensure consistent compaction density. Regular thickness measurements during rolling are necessary to prevent pressure fluctuations.

Longitudinal Thickness Variation (Rebound): After rolling, the sheet may meet thickness requirements, but thickness increase may occur during cutting due to rebound. This is caused by excessive internal moisture in the sheet or too fast rolling speed.
Solutions:

Adopt hot rolling processes:
① Remove internal moisture from the electrode.
② Reduce material rebound after rolling.
③ Enhance adhesion between active materials and the current collector due to material softening in a molten state during heating.
④ Reduce internal stress to minimize defects caused by stress release during cutting or die-cutting.
⑤ Lower the deformation resistance of the electrode, improving the liquid absorption capacity of active materials.

4. Severe Wavy Edges

After coating, part of the copper/Aluminum Foil is exposed. After rolling, dense wavy patterns may appear on the exposed foil edges, affecting processes such as cell winding, hot pressing, and tab welding.
Causes: Copper and aluminum foils, produced by extruding copper/aluminum blocks, have good malleability and ductility. During rolling, active materials are compressed, exerting pressure on the foil and causing elongation. The uncoated foil edges do not elongate, while the coated areas do, resulting in wavy wrinkles at the edges. Active materials on wrinkled foils are prone to cracking, bulging, or shedding.
Solutions:

Optimize the design of areal density and compaction density.

Ensure the rolls are smooth (uneven rolls can also cause wavy edges).